Because it's objectively easier to do cross-platform GUI development using a single, well-defined and portable API that someone else maintains.
That that API is nowadays de-facto WebKit supporting a HTML/CSS/JS stack is a natural, but unfortunate choice, and it's got a much lower barrier of entry and much less maintenance overhead than, say, Gtk+ or Qt.
Because it's objectively easier to do cross-platform GUI development using a single, well-defined and portable API that someone else maintains.
That that API is nowadays de-facto WebKit supporting a HTML/CSS/JS stack is a natural, but unfortunate choice, and it's got a much lower barrier of entry and much less maintenance overhead than, say, Gtk+ or Qt.